Prosthetic blood circulation device having a pyrolytic carbon coated blood contacting surface

ABSTRACT

A prosthetic device for implantation in or use with a living body. A substrate is coated with impermeable pyrolytic carbon which provides an inert and antithrombogenic outer surface. The conditions at which the pyrolytic carbon is deposited are controlled to match the thermal coefficient of expansion of the pyrolytic carbon to that of the substrate and to provide a strong carbon which contributes substantial structural strength to the composite prosthetic device. The carbon is preferably isotropic and may be doped with a suitable carbide-forming element, such as silicon, to provide additional structural strength and wear resistance. Devices having such coatings on the portions coming in contact with blood are valuable for extracorporeal circulation of the bloodstream of a human patient.

[ 51 Aug. 22, 1972 United States Patent Bokrosetal.

1541 PROSTHETIC BLOOD CIRCULATION OTHER PUBLICATIONS Construction of aRigid- Case, Double Ventricle Artificial Heart" DEVICE HAVING APYROLYTIC by S. R. Topaz et al., Transactions American Society forArtificial Internal Organs, Vol. XIII, Apr. 1967, pp. 294- 298.

mm m s T AG ON n NC n us A0 CC [72] Inventors: Jack C. Bokros, SanDiego; Willard H. Ellis, Leucadia, both of Calif.

[73] Assignee: Gulf General Atomic Incorporated,

The Coating of lntravascular Plastic Prostheses with Coloidal Graphiteby V. L. Gott et al 50, No.

Surgery, Vol. 2, pp. 382- 389, Aug. 1961.

San Diego, Calif.

July 28, 1970 Primary ExaminerRichard A. Gaudet Assistant ExaminerRonaldL. Frinks [22] Filed:

Appl' 58321 Att0rneyAnderson, Luedeka, Fitch, Even and Tabin Related US.Application Data [63] Continuation-in-part of Ser. No. 649,811, June[57] ABSTRACT A prosthetic device for implantation in or use with a 29,1967, Pat. No. 3,526,005, Continuation-inpart of Ser. No. 821,080, May1, 1969. living body. A substrate is coated with impermeable pyrolyticcarbon which provides an inert and antithrombogenic outer surface. Theconditions at which 3/ 117/46 128/1 R the pyrolytic carbon is depositedare controlled to [51] Int. 1/22, A6lf l/24 match the thermalcoefficient of expansion of the era ac H m c s mcm iD. a UL-1C0 OP d hF.5d 81 m ..0 Ch S du.wbcn l nS Uena a mw vt efi m eA m [IS t 0.1 o car. Id a 0 pnm h in u b bfie UH CH MC 80 9 -Dh CO gU h mn t thmti. w OfIEOm ohc n O C twe v xfmwmm t mn wkmb m mormdwmu O n w ae C mhcw m ncfim d w k wmm m .l.bonm n a a a im. an w m .w mm om m ,lvtJamcatrmmm 08Chn m w.. wwmmmnoemmm PvMTw w 64 G06 3 9 CCC3 new 666 m DC 444 2 777 249HHHW nn s mmmm D T. lnm ,m g N "3" m m aaa G w A ttuo. l 3 t. 66 l DA2.l P SSMWZ C C S 0mm H 6 E hku 3 c T 000 m A BBBD H w 2 f h m u e D 7880R E 6667 a T 9999 el l HHHH W N 929 2 U d d 906 .l 2620 F 9 999 8996 l l9962 8 6 23 .61 U 3333 3,330,698 11/1967 Podolsky 1 17/46 CG 14 Claims,8 Drawing Figures PATENTEDwsza I972 SHEET 1 0F 2 ZZZ/ INVENTORS ATTYS.

PATENTEDwszz m2 SHEET 2 0F 2 X Z m? Wad PROSTI-IETIC BLOOD CIRCULATIONDEVICE HAVING A PYROLYTIC CARBON COATED BLOOD CONTACTING SURFACE Thisapplication is a continuation-in-part of our earlier patent applicationsSer. No. 649,811, filed June 29, 1967 now US. Pat. No. 3,526,005 andSer. No. 821,080, filed May 1, 1969.

This invention relates generally to prosthetic devices and moreparticularly to prosthetic devices for use within a living body or inassociation therewith.

Prosthetic devices, such as intravascular prostheses, have been used fora number of years, and it is expected that usage of such devices willincrease in the future as medical expertise continues to improve. Oneexample is the artificial heart valve which is used fairly extensivelytoday, and more complex circulatory assist devices, including thosewhich are used extracorporeally, are currently under development.Artificial kidneys are another class of prosthetic devices becoming moreand more available.

In order to further the development and utilization of prostheticdevices, the surfaces of these devices which come in contact with bloodand tissue should be completely compatible therewith, whether thecontact be made by implantation within or insertion into the body or bypassage therethrough of blood at locations exterior of the body. Two ofthe most common materials for intravascular prosthesis are metals, forapplications where high strength and good wearability are important, andplastics for applications wherein flexibility is needed. Metals arethrombogenic and are subject to corrosion. Plastics, without sometreatment, are also thrombogenic and are subject to degradation.Stainless steel and tantalum are among the most popular metals usedtoday, whereas polyethylene, Teflon and the polycarbonates are examplesof plastics considered suitable. None of these materials are consideredto be totally satisfactory for the construction of prosthetic devices.

It is an object of the present invention to provide improved prostheticdevices by utilizing improved materials of construction. Another objectis to provide prosthetic devices which are nonthrombogenic and whichwill retain this characteristic although implanted in the body for longperiods of time. A further object is to provide improved prostheticdevices which are compatible with body tissue, do not cause irritationthereof, and have good strength and resistance to deterioration whenimplanted within or inserted into a living body. Still another object isto provide a method for making improved prosthetic devices, particularlyfor extracorporeal use. One further object is to provide improved partsfor use in extracorporeal apparatus which is exposed to the circulationof blood.

These and other objects of the invention should be clearly apparent fromthe following description of devices embodying various features of theinvention when read in conjunction with the accompanying drawings,wherein:

FIG. 1 is section view through a circulatory assist device;

FIG. 2 is an enlarged perspective view of a valve disc used in thedevice shown in FIG. 1;

FIG. 3 is a sectional view of another type of circulatory assist device;

FIG. 4 is a perspective view of still another circulatory assist device;

FIG. 5 is a plan view of yet another type of circulatory assist device;

FIG. 6 is a sectional view taken along line 6-6 of FIG. 5;

FIG. 7 is a perspective view of an alternative type of impeller that isemployed in the general type of circulatory assist device shown in FIGS.5 and 6; and

FIG. 8 is a perspective view of a cannula of the type which might beinserted into the circulatory system of the human body to facilitate useof some extracor poreal type of device.

It has been found that prosthetic devices having improvedcharacteristics can be made by coating suitable substrates of thedesired shape and size with dense pyrolytic carbon. Dense pyrolyticcarbon has been found not only to significantly increase the strength ofthe substrate upon which it is coated, but also to resist wear anddeterioration even if implanted within a living body for long periods oftime. While reference is hereinafter generally made to the use of theprosthetic devices in combination with a human body, it should also berecognized that the improved prosthetic devices may be used in otherliving mammals. For example, it may be desirable to use pins whichinclude the indicated pyrolytic carbon coatings for use in repairing orsetting broken bones in horses or dogs. Moreover, for purposes of thisapplication, the term prosthetic device is intended to include parts forextracorporeal devices which will be in contact with the bloodstream ofa living person.

For use on complex shapes and in order to obtain maximum strength, it isdesirable that the pyrolytic carbon be nearly isotropic. Anisotropiccarbons, though thrombo-resistant, tend to delaminate when complexshapes are cooled after coating at high temperatures. Thus, for coatingcomplex shapes (i.e., those having radii of curvature less thanone-quarter inch), the pyrolytic carbon should have a BAF (BaconAnisotropy Factor) of not more than about 1.3. For noncomplex shapes,higher values of BAF up to about 2.0 may be used, and for flat shapes,pyrolytic carbon having a BAF as high as about 20 may be used. The BAFis an accepted measure of preferred orientation of the layer planes inthe carbon crystalline structure. The technique of measurement and acomplete explanation of the scale of measurement is set forth in anarticle by G. E. Bacon entitled A Method for Determining the Degree ofOrientation of Graphite which appeared in the Journal of AppliedChemistry, Volume 6, page 477 1956). For purposes of explanation, it isnoted that 1.0 (the lowest point on the Bacon scale) signifies perfectlyisotropic carbon.

In general, the thickness of the outer pyrolytic carbon coating shouldbe sufficient to impart the necessary stress and strain fracturestrengths to the particular substrate being coated, and usually thecoating will be at least about 50 microns thick. If a fairly weaksubstrate is being employed, for instance one made of artificialgraphite, it may be desirable to provide a thicker coating of pyrolyticcarbon to strengthen the composite prosthetic device. Moreover, althoughan outer coating which is substantially entirely isotropic pyrolyticcarbon has adequate structural strength, the codeposition of silicon orsome similar carbide-forming additive improves the strength and wearresistance of the carbon coating. As described in more detailhereinafter, silicon in an amount up to at least about 20 weight percentcan be dispersed in SiC throughout the pyrolytic carbon withoutdetracting from the desirable thrombo-resistant properties of thepyrolytic carbon.

The density of the pyrolytic carbon is considered to be an importantfeature in determining the additional strength which pyrolytic carboncoating will provide the substrate. The density is further important inassuring that the pyrolytic carbon surface which will be exposed toblood in the environment wherein it will be used is smooth andsubstantially impermeable. Such surface characteristics are believed toreduce the tendency of blood to coagulate on the surface of theprosthetic device. It is considered that the pyrolytic carbon should atleast have a density of about L5 grams per cm and such pyrocarbon isreferred to in this application as dense.

A further characteristic of the carbon which also affects the strengthcontribution thereof is the crystallite height or apparent crystallitesize. The apparent crystallite size is herein termed L and can beobtained directly using an X-ray diffractometer. In this respect L =0.89lt/B cost) wherein:

A is the wave length in A.

B is the half-height (002) line width, and

0 is the Bragg angle Pyrolytic carbon coatings for use in prostheticdevices should have a crystalline size no greater than about 200A. Ingeneral, the desirable characteristics of pyrolytic carbon for use inprosthetic devices are greater when the apparent crystallite size issmall, and preferably the apparent crystallite size is between about andabout 50A.

Because the substrate material for the prosthetic device will often becompletely encased in pyrolytic carbon, or at least will have one of itssurfaces covered with pyrolytic carbon at a location that will be incontact with either body tissue or the blood, choice of the materialfrom which to form the substrate is not of utmost importance. Forexample, if the particular prosthetic device is a pin or a small tube ora portion of a valve, it is likely that the prosthetic device would becompletely covered with pyrolytic carbon. However, for purposes of thisapplication, the term prosthetic device is also used to include a partof an apparatus which is used exterior of the body, for example, as apart of an auxiliary blood pump or circulatory assist device; and forsuch a part, it may be necessary to coat only the surfaces which come incontact with the blood.

It is considered very important that the substrate material becompatible with pyrolytic carbon, and more particularly suitable for useat the process conditions for coating with pyrolytic carbon. Although itis desirable that the substrate material have good structural strengthto resist possible failure during its end use, materials which do nothave high structural strengths may be employed by using the pyrolyticcarbon deposited thereupon to supply additional structural strength forthe prosthetic device.

Pyrolytie carbon is, by definition, deposited by the pyrolysis of acarbon-containing substance so the substrate will be subjected to thefairly high temperatures necessary for pyrolysis. Generally,hydrocarbons are employed as the carbon-containing substance to bepyrolyzed, and temperatures of at least about l,0OO C. are used. Someexamples of the deposition of pyrolytic carbon to produce coatedarticles having increased stability under high temperature and neutronirradiation conditions are set forth in U.S. Pat. No. 3,298,921.Processes illustrated and described in this U.S. Pat. employ methane asthe source of carbon and utilize temperatures generally in the rangefrom about 1500 to 2,300 C. Although it may be possible to depositpyrolytic carbon having the desired properties with regard to theinstant invention at somewhat lower temperatures by using otherhydrocarbons, for example, propane or butane, generally it is consideredthat the substrate material should remain substantially unaffected bytemperatures of at least about l,0O0 C., and preferably by even highertemperatures.

Because the substrate is coated at relatively high temperatures and theprosthetic device will be employed at temperatures usually very close toambient, the coefficients of thermal expansion of the substrate and ofthe pyrolytic carbon deposited thereupon should be relatively close toeach other if the pyrolytic carbon is to be deposited directly upon thesubstrate and a firm bond therebetween is to be established. Whereas inthe aforementioned U.S. Pat. there is description of the deposition ofan intermediate low density pyrolytic carbon layer, the employment ofwhich might provide somewhat greater leeway in matching the coefficientsof thermal expansion, it is preferable to deposit the pyrolytic carbondirectly upon the substrate and therefor avoid the necessity for such anadditional intermediate layer. Pyrolytic carbon having the desiredcharacteristics can be deposited having a thermal coefficient ofexpansion in the range of between about 3 and about 6 X 10' C. measuredat 20 C. Accordingly, substrate materials are chosen which have theaforementioned stability at high temperatures and which have thermalcoefficients of expansion within or slightly above this general range,for example up to about 8 X l0' C. Examples of suitable substratematerials include artificial graphite, boron carbide, silicon carbide,tantalum, molybdenum, tungsten, and various ceramics, such as mullite.

The pyrolytic carbon coating is applied to the substrate using asuitable apparatus for this purpose. Preferably, an apparatus isutilized which maintains the substrate in motion while the coatingprocess is carried out to assure that the coating is uniformlydistributed on the desired surfaces of the substrate. A rotating drumcoater or a vibrating table coater may be employed. When the substratesto be coated are small enough to be levitated in an upwardly flowing gasstream, a fluidized bed coater is preferably used.

As discussed in detail in the aforementioned U.S. Pat, thecharacteristics of the carbon which are deposited may be varied byvarying the conditions under which pyrolysis is carried out. Forexample, in a fluidized bed coating process wherein a mixture of ahydrocarbon gas, such as methane, and an inert gas, such as helium orargon, is used, variance in the volume percent of methane, the totalflow rate of the fluidizing gas stream, and the temperature at whichpyrolysis is carried out all affect the characteristics of the pyrolyticcarbon which is deposited. Control of these various operationalparameters not only allows deposition of pyrolytic carbon having thedesired density, apparent crystallite size, and isotropy, but alsopermits the regulation of the desired thermal coefficient of expansionwhich the pyrolytic carbon has. This control also allows one to grade acoating in order to provide a variety of exterior surfaces. For example,a highly oriented surface coating is believed to provide enhancedthromboresistance which may be desirable for certain applications. Onecan deposit a strong base isotropic pyrocarbon coating, having a BAF of1.3 or less, and near the end of the coating operation, one cangradually change the coating conditions to obtain a highly orientedouter layer. Using this technique, suitable coatings having outersurfaces which are highly anisotropic and, for example, are about 25microns thick, can be conveniently deposited.

Generally, when pyrolytic carbon is deposited directly upon the surfaceof the substrate material, the pyrolysis conditions are controlled sothat the pyrolytic carbon which is deposited has a coefiicient ofexpansion matched to within about plus or minus 50 percent of thesubstrate materials thermal coefficient of expansion, and preferably towithin about plus or minus 20 percent thereof. Because pyrolytic carbonhas greater strength when placed in compression than when placed intension, the thermal coefficient of expansion of the pyrolytic carbonmost preferably is about equal to or less than that of the substrate.Under these condition, good adherence to the substrate is establishedand maintained during the life of the prosthetic devices.

As previously indicated, the coating may be substantially entirelypyrolytic carbon, or it may contain a carbide-forming additive, such assilicon, which has been found to increase the wear resistance andoverall structural strength of the coating. Silicon in an amount of upto about 20 weight percent, based upon total weight of silicon pluscarbon, may be included without detracting from the desirable propertiesof the pyrolytic carbon, and when silicon is used as an additive, it isgenerally employed in an amount between about and weight percent.Examples of other carbide-forming elements which might be used asadditives in equivalent weight percents include boron, tungsten,tantalum, niobium, vanadium, molybdenum, aluminum, zirconium, titaniumand hafnium. Generally, such an element would not be used in an amountgreater than 10 atom percent, based upon total atoms of carbon plus theelement.

The carbide-forming additive is codeposited with the pyrolytic carbon byselecting a volatile compound of the element in question and supplyingthis compound to the deposition region. Usually, the pyrolytic carbon isdeposited from a mixture of an inert gas and a hydrocarbon or the like,and in such an instance, the inert gas is conveniently employed to carrythe volatile compound to the deposition region. For example, in afluidized bed coating process, all or a percentage of the fluidizing gasmay be bubbled through a bath of methyltrichlorosilane or some othersuitable volatile liquid compound. Under the temperature whereat thepyrolysis and codeposition occurs, the particular element employed isconverted to the carbide form and appears dispersed as a carbidethroughout the resultant product. As previously indicated, the presenceof such a carbide-forming additive does not significantly change thecrystalline structure of the pyrolytic carbon deposited from that whichwould be deposited under the same conditions in the absence of such anadditive.

Pyrolytic carbon having the physical properties mentioned hereinbefore,is considered to be particularly advantageous for constituting thesurface for a prosthetic device because it is antithrombogenic and isinert to the metabolic processes, enzymes, and other juices found withinliving bodies. The antithrombogenic properties of pyrolytic carbon arebelieved to be dependent upon its sterility and the removal of allchemisorbed oxygen therefrom. Before use, the device may be sterilized,for example, by heating in a suitable vacuum for about 6 hours at aboutC. or by steam autoclavmg.

As an alternative to the foregoing sterilization and degassingtechniques, the prosthetic devices can be sterilized in benzalkoniumchloride and then treated with a suitable anticoagulant which safeguardsagainst the occurrence of thrombosis. An anticoagulant such as heparincan be used. Application may be simply made by soaking the prostheticdevice in benzalkonium chloride and then in a heparin solution. Asuitable heparin solution may be prepared by mixing l0 mgs. of heparinper ml. of saline, saline being a solution of sodium chloride in water.The sorption of heparin by pyrolytic carbon surfaces purposely preparedwith accessible porosity at the outer surface thereof is improved bypretreatment with a cationic, surface-active agent such as an aqueoussolution of benzalkonium chloride and heparin. It should be kept inmind, however, that impermeable pyrolytic carbon is inherentlythromboresistant and prior treatment with heparin is not essential.

When the prosthetic device is ready for its intended use, for example asa part of apparatus that will function exterior of a living body, orperhaps as an implant within a living body to repair an intravasculardefect, known surgical procedures or the like are employed. A pyrolyticcarbon-coated device may be secured in the proper location within thebody, for example, by joining with Dacron cloth and appropriatelysuturing using standard suturing methods.

Illustrated in FIG. 1 of the drawings is a circulatory assist device inthe form of an air operated pump 11. The pump 11 has a body 13 with aninlet 15 and an outlet 17 for blood and having an opening 19 forconnection to an air line 21. A flexible bladder 23 disposed within thepump body provides a pumping chamber 25 which is closed at opposite endsby an inlet valve 27 and an outlet valve 29. Each of the valves 27, 29include doubly convex-shaped disc 31 which is proportioned to close thevalve opening therethrough and which is maintained in association withthe opening by a retainer 33. Each disc valve element 31, shown in FIG.2, is formed with two identical convex surfaces.

As one example of using this pump 11, the pump inlet 15 is connected tothe left ventricle of the heart. In FIG. 1, the pump 11 is shown withthe inlet valve 27 in the open position so that blood flowing from theleft ventricle during systole flows into the flexible bladder 23. Duringthis filling cycle, the air line 21 connected to the opening 19 isvented. The outlet 17 from the pump 11 is connected to the descending orthoracic aorta. During the filling cycle, the outlet valve 29 in thepump is closed (as shown) as the result of the pressure in the aorta.Subsequently, an external control system supplies air pressure throughthe opening 19 to the region between the body 13 and the flexiblebladder 23. The application of air pressure squeezes the bladder 23closing the inlet valve 27, opening the outlet valve 29 and ejecting theblood from the bladder into the descending aorta.

It is most important that thrombosis be avoided which might result inclotting and eventual deterioration in the performance of such a pump11. The movable valve discs 31 are one of the locations most susceptibleto thrombosis, and it has been found that by providing these valve discswith exterior coatings of pyrolytic carbon, excellent resistance tothrombosis is provided. The discs 31 may be made of graphite, machinedto shape and coated with a SO-micron thick coating of dense pyrocarbon.The disc retainers 31 are also advantageously coated with pyrocarbon.Valves using such discs continue to open and close well over longperiods of use for pumping human blood.

Shown in FIG. 3 is another type of circulatory assist device or pump 41which also utilizes compressed air or the like to power the pumpingoperation. The pump includes an outer body 43 having formed therein acentral cylindrical section 45, an upper dish-shaped section 47 and alower dish-shaped section 49. A movable pumping element 51 has thegeneral shape of an inverted funnel. The tubular stem portion 53 extendsthrough a central opening in the upper body section 47 and surmounts aconcave-shaped portion 55 that is contoured similarly to the internalsurface of the upper pump body section. A lower inlet 57 is provided inthe pump body 43 through which flow is regulated by a pivoting valveelement 59. The movable pumping element 51 carries another pivoted valveelement 59 in the stem portion 53 thereof which serves as the outletvalve. A passageway 61 is provided in the upper surface portion 47 ofthe pump body which is adapted for connection by a suitable conduit to acontrol mechanism (not shown).

The pump 41 may be connected in the same manner as the pump 11illustrated in FIG. 1. 1n the position shown, the lower inlet valveelement 59 is in open position and blood is flowing into the pumpingchamber defined generally between the lower dish-shaped section 49 ofthe pump body 43 and the movable pumping element 51. The blood pressurein the aorta maintains the upper valve element 59 in closed position,and the movable pumping element 51 reciprocates upward with the inflowof the blood. During the filling phase, the region between the uppersurface of the movable pump element 51 and the concave undersurface ofthe upper pump body section 47 is vented via the passageway 61.Subsequently, air pressure is applied through the passageway 61 to drivethe movable pump element 51 downward. This action closes the lower inletvalve, opens the outlet vaive and discharges blood from the pumpingchamber into the descending aorta.

Preferably, all of the internal surfaces of the pump 41 which come incontact with blood are coated with pyrolytic carbon. In this respect,the internal surfaces of the sections 45 and 49 of the pump body 43would be so coated along with the inner surface of the inlet 57. Theentire inner surface of the movable pumping element 51 should also becoated. Likewise, both of the pivoting valve elements 59 are completelycoated with a layer of pyrolytic carbon. In addition to beingthromboresistant, the pyrolytic carbon provides an excellent bearingsurface and exhibits good wear characteristics in the region of thecylindrical wall section 45 where there is sliding contact with theperipheral edge of the reciprocating pumping element 51. Such a pump 41is capable of continuous operation without the development of bloodclotting.

Illustrated in FIG. 4 is another type of circulatory assist device inthe form of a piston-type pump 71. The pump has an outer body or casing73 and contains a sleeve 75 that serves as a cylinder wall that is insliding contact with a floating piston 77 which has the shape of a rightcircular cylinder. The piston 77 slides freely in the sleeve 75, and itsmovement is controlled via an opening 79 in the lower surface of thecasing 75 to which a conduit is connected, as the case of the pumps 1 1and 41. The casing 73 forms a pumping chamber 81 above the upper face ofthe piston 77 and contains, side-by-side, an inlet 83 and an outlet 85,each of which are provided with ball valves 87 and 89, respectively.Each valve includes a movable spheroid 91 and a retaining cage 93.

The operation of the pump 71 is similar to the operation previouslydescribed, and the pump is illustrated near the end of the pumpingphase, just before the filling phase begins. The floating piston 77moves downward when blood is flowing into the pumping chamber 81 throughthe inlet valve opening, and the blood pressure in the aorta maintainsthe outlet ball 91 in the closed position. Upon completion of thefilling phase, air pressure is applied to the lower opening 79, forcingthe floating piston 77 upward, closing the inlet valve 87 and pumpingthe blood from the pumping chamber 81 through the outlet valve 89 intothe descending aorta.

It has been found that this piston blood pump 71 has substantiallyimproved resistance to blood clotting if the sleeve 75 and the piston 77are coated with an exterior layer of pyrolytic carbon. Moreover, themovable ball valve spheroids 91 are also advantageously made from asuitable substrate, such as graphite, and coated with pyrolytic carbon.Depending upon the material from which the retaining members 93 aremade, these members are also provided with an outer coating of pyrolyticcarbon that prevents clotting thereadjacent over a long duration ofoperation.

Shown in FIGS. 5 and 6 in a centrifugal type of circulatory assistdevice or pump 101 having a two-piece housing wherein a rotor 103revolves. An upper por tion 105 of the housing flares outward from acentral inlet opening 107 to present a smooth flaring undersurface whichmay be described as being generally bellshaped. A lower housing portion109 mates with the upper portion 105 and contains a flat circular wall 11 1 having an upstanding short peripheral wall 1 13.

The rotor 103 consists of three separate sections 115, 117 and 119 eachhaving progressively slightly greater curvature than the underside ofthe upper housing portion 105 which is interconnected by pins 121. Thelowermost rotor section 119 is linked by suitable struts 123 to acentral shaft 125 which extends downward through a drilled hole in thecircular wall 111 to facilitate connection to an electric motor 127. Thelower portion 109 of the two-piece housing contains a tangentiallylocated outlet 129 in the peripheral wall 113. Spacing between the threerotor sections is such as to provide a viscous drag on the blood andimpart centrifugal motion to it which propels it outward and throughoutlet 129. Accordingly, revolution of the three-piece rotor 103 by theelectric motor 127 causes blood to be drawn into the inlet opening 107and centrifugally discharged through the tangential outlet 129.

It has been found that the performance of the centrifugal pump 101 issubstantially improved by the avoidance of clotting as a result ofcoating the components that come in contact with blood with a layer ofpyrolytic carbon. In this respect, the interior surface of the pumpingcavity formed by the two-piece housing is coated with pyrolytic carbon.Moreover, all of the surfaces of the three-segment rotor 103 and theconnecting pins 121 and struts 123 are also coated.

Shown in FIG. 7 is an alternative design of a rotor 131 which is alsoemployed in a centrifugal circulatory assist device of the general typeas that shown in FIGS. and 6. The rotor 131 is afiixed to a drive shaft133 attached to it and has an upper conical portion 135 from whichextend six triangular-shaped blades 137. The entire rotor and anyportion of the shaft which extends into the pumping cavity arepreferably coated as a unit with a layer of pyrolytic carbon in themanner hereinbefore described.

Illustrated in FIG. 8 is a cannula 141 of tee shape. The cannula 141 isdesigned for implantation in the body of a patient who will periodicallybe submitted to artificial kidney treatments. For example, the longstraight run 143 of the tee may be spliced into the vein of a patientwhile the short stem section 145 of the tee extends upward to thesurface of the skin. If pyrolytic carbon is used to completely coat thecannula 141, it can be implanted as a permanent installation inasmuch asclotting is avoided. Normally, the short stern section 145 of the tee isclosed by a suitable plug, and the blood flows straight through the runof the tee. When, for example, dialysis is desired, blood is removedfrom an artery using a similar tee and is returned to the vein via thestem 145. The ability of a pyrolytic carbon coating to permit apermanent installation of this type is of substantial advantage to apatient who must frequently be subjected to such treatments.

The following examples illustrate several coating processes forproducing prosthetic devices having pyrolytic carbon surfaces exhibitingvarious advantages of the invention. Although these examples include thebest modes presently contemplated by the inventors for carrying outtheir invention, it should be understood that these examples are onlyillustrative and do not constitute limitations upon the invention whichis defined by the claims appearing at the end of this specification.

EXAMPLE I Short tubes are constructed of artificial graphite each havinga length of 9mm., an internal diameter of 7mm. and a wall thickness of0.5mm. The artificial graphite employed has a coefficient of thermalexpansion of about 4 X 10' C. when measured at 50 C. The short tubes arecoated with pyrolytic carbon using a fluidized bed coating apparatus.

The fluidized bed apparatus includes a reaction tube having a diameterof about 3.8 cm. that is heated to a temperature of about l,350 C. Aflow of helium gas sufficient to levitate the relatively small tubes ismaintained upward through the apparatus. The small short tubes arecoated together with a charge of zirconium dioxide particles of about 50grams, which particles have diameters in the range of about 150 to 250microns. The particles are added along with the short tubes to provide adeposition surface area of the desired amount, relative to the size ofthe region of the reaction tube wherein pyrolysis occurs, inasmuch asthe relative amount of available surface area is another factor whichinfluences the physical characteristics of the resultant pyrolyticcarbon.

When the temperature of the articles which are levitated within thereaction tube reaches about 1350 C., propane is admixed with the heliumto provide an upwardly flowing gas stream having a total flow rate ofabout 6,000 cc. per minute and having a partial pressure of propane ofabout 0.4 (total pressure one atmosphere). The propane decomposes underthese conditions and deposits a dense isotropic pyrolytic carbon coatingupon all of the articles in the fluidized bed. Under these coatingconditions, the carbon deposition rate is about 5 microns per minute.The propane gas flow is continued until an isotropic pyrolytic carboncoating about 200 microns thick is deposited on the outside of thetubes. At this time, the propane gas flow is terminated, and the coatedarticles are cooled fairly slowly in the helium gas and then removedfrom the reaction tube coating apparatus.

The short tubes are examined and tested. The thickness of the pyrolyticcarbon coating on the interior of the tube measures about 200 microns.The density of the isotropic carbon uniformly is found to be about 2.0grams per cm. The BAF is found to be about 1.1. The apparent crystallitesize is measured and found to be about 30 to 40A. Mechanical tests ofthe coated short tubes are made to determine their strength incomparison to additional uncoated graphite tubes. The crushing load ofthe uncoated graphite tubes, loaded parallel to the diameter, is foundto be about 4 pounds. The crushing load of the coated tubes is about 25pounds, about 6 times higher. Another of the coated tubes is sterilizedby heating to about 1000C. in a vacuum and then is soaked for 15 minutesin a dilute solution of benzalkonium chloride (1 part by 1,000 partswater). The coated tube is then removed, rinsed and then soaked for 15minutes in a heparin solution prepared at a level of 10 mgs. of heparinper ml. of saline. After removal, the tube is rinsed ten times withsaline and is then tested with blood. After contact with blood for about24 hours, no sign of clotting is shown, and clotting normally occurswithin a matter of minutes. The pyrolytic carbon-coated, graphitesubstrate articles are considered to be excellently acceptable for useas prosthetic devices within the body of human beings.

EXAMPLE II A number of short tubes having the same dimensions as thoseused in Example I but made of tantalum are provided. Tantalum has athermal coefficient of expansion of about 6.5 X l0 C., measured at 20 C.The short tubes are coated in the fluidized bed reaction tube employedin Example I. In order to match the pyrolytic carbon coefficient ofthermal expansion to that of the tantalum substrate, a coatingtemperature of l,600 C. is employed using a 15 percent propane 85percent helium gas stream having a total flow rate of about 6,000 cc.per minute. The short tubes are levitated together with a similar 50gram charge of particles of zirconium dioxide at atmospheric pressure.Deposition of pyrolytic carbon is carried out for about 20 minutes,after which period a layer of isotropic pyrolytic carbon about 150microns thick coats the outer surface of each of the tubes. At the endof this time the propane flow is discontinued, and the coated tubes arecooled and removed from the reaction tube.

Examination and testing shows that the density of the isotropicpyrolytic carbon deposited is about 1.6 grams per cm.*. The BAF is about1.0. The apparent crystallite size is between about 50 to 60A. Thethermal coefficient of expansion of the pyrolytic carbon measures aboutX l0' C. at about 20 C. Mechanical testing of the coated tubes showsthat the strength and wearability is acceptable and that the coating isfirmly affixed to the substrate.

One of the coated short tubes is sterilized and treated as in Example Iexcepting that the treatment with benzalkonium chloride and heparin isomitted. The tube is tested with blood, and there is no sign of clottingafter contact therewith for 24 hours. The carboncoated tantalum articlesare considered to be excellently acceptable for use as a part of aprosthetic device for implantation within a human body.

EXAMPLE III A number of short tubes having the same dimensions as thoseused in Example I but made of tungsten are provided. Tungsten has athermal coefficient of expansion of about 4.4 X 10 C., measured at 27 C.The short tubes are coated in the fluidized bed reaction tube employedin Example I. In order to match the pyrolytic carbon coefficient ofthermal expansion to that of the tungsten substrate, a coatingtemperature of 1600C. is employed using a percent propane 85 percenthelium gas stream having a total flow rate of about 6,000cc. per minute.The short tubes are levitated together with a similar 50 gram charge ofparticles of zirconium dioxide. Deposition of pyrolytic carbon iscontinued for about minutes, at which time a layer of isotropicpyrolytic carbon about 150 microns thick coats the outer surface of eachof the tubes. The propane flow is discontinued, and the coated tubes arecooled and removed from the reaction tube.

Examination and testing shows that the density of the isotropicpyrolytic carbon deposited is about 1.6 grams per cc. The BAF is about1.0. The apparent crystallite size is between about 50 to 60A. Thethermal coefficient of expansion of the pyrolytic carbon measures about5 X l0 C. at about 20 C. Mechanical testing of the coated tubes showsthat the strength and wearability is acceptable and that the coating isfirmly affixed to the substrate.

One of the coated short tubes is sterilized and treated as in Example Iwith benzalkonium chloride and heparin and tested with blood. There isno sign of clotting after contact therewith for 24 hours. Thecarbon-coated tungsten articles are considered to be excellentlyacceptable for use as a part of a prosthetic device for implantationwithin a human body.

EXAMPLE IV A number of short tubes having the same dimensions as thoseused in Example I but made of molybdenum are provided. Molybdenum has athermal coefficient of expansion of about 5.3 X l0' C., measured at 20C. The short tubes are coated in the fluidized bed reaction tubeemployed in Example I. In order to match the pyrolytic carboncoefficient of thermal expansion to that of the molybdenum substrate, acoating temperature of l,350 C. is employed using a 30 percent propanepercent helium gas stream having a total flow rate of about 5,500cc. perminute. The short tubes are levitated together with a similar 50 gramcharge of particles of zirconium dioxide. Deposition of pyrolytic carbonoccurs, and after about 30 minutes a layer of isotropic pyrolytic carbonabout I50 microns thick coats the outer surface of each of the tubes. Atthe end of this time, the propane flow is discontinued, and the coatedtubes are cooled and removed from the reaction tube.

Examination and testing shows that the density of the isotropicpyrolytic carbon deposited is about 2.0 grams per cm. The BAF is about1.1. The apparent crystallite size is between about 30 and 40A. Thethermal coefficient of expansion of the pyrolytic carbon measures about5 X 10 C. at about 20 C. Mechanical testing of the coated tubes showsthat the strength and wearability is acceptable and that the pyrolyticcarbon coating is firmly bonded to the substrate.

One of the coated short tubes is polished, sterilized and treated as inExample I with benzalkonium chloride and heparin and is tested withblood. There is no sign of clotting after contact therewith for 24hours. The carbon-coated molybdenum short tubes are considered to beexcellently acceptable for use as a part of a prosthetic device forimplantation within a human body.

EXAMPLE V A number of graphite tubes having the same characteristics anddimensions as those used in Example I are introduced into a reactiontube which is about 6.3 cm. in diameter, together with an ancillarycharge of grams of zirconium oxide spheroids having an average particlesize of about 400 microns. A fluidizing flow of helium is fed upwardthrough the reaction tube as the temperature of the small tubes andparticles is raised to about l,350C. When this temperature is reached,propane is admixed with the helium to provide a total gas flow of about8,000 cc. per minute, having a partial pressure of propane of about 0.4atm.(total pressure of 1 atm.). All of the helium is bubbled through abath of methyltrichlorosilane at about room temperature. The propane andthe methyltrichlorosilane pyrolyze to deposit a mixture of isotropiccarbon and silicon carbide on the small tubes, and the coating processis continued until a coating about l2 mils (300 microns) thick isobtained, a time of about an hour.

The resultant coated tubes are allowed to cool to ambient temperature,and they are then removed from the reaction tube. Examination of theisotropic carbon-sil icon carbide coating shows that it has acoefficient of thermal expansion of about 6 X 10/ C. and a density of 2grams per cm The coating contains about 10 weight percent silicon (basedupon total weight of silicon plus carbon) in the form of siliconcarbide. The isotropic carbon has a BAF of about 1.1 and an apparentcrystallite size of about 35A. Mechanical testing of the coated tubesshows that the strength and wearability are fully acceptable and thatthere is a firm bond between the coating and the graphite substrate.

One of the coated tubes is polished, sterilized and treated as inExample I, using benzalkonium chloride and heparin, and it is thentested with blood. There is no sign of clotting after contact with bloodfor 24 hours. The tubes which are coated with pyrolytic carboncontaining the silicon carbide additive are considered to be excellentlyacceptable for use as a part of a prosthetic device and suitable forimplantation within a human body.

Although the examples have been particularly directed to the coating anduse of short tubes, it should be understood hat the examples areprovided for the purpose of illustration. Any suitably shaped elementsincluding all of those shown in the drawings can be coated to provideprosthetic devices of the improved design. Deposition of pyrolyticcarbon in a fluidized bed process is excellently suited for the smoothcoating of even very complex-shaped elements. The improved prostheticdevices have excellent resistance to degradation in a living body andare likewise eminently well suited for parts in circulatory assistdevices that handle the bloodstream of a living human being.

Various of the features of the invention are set forth in the claimsthat follow.

What is claimed is:

l. A prosthetic device designed for use in the bloodstream circulationof a human being, which device comprises a substrate having a shape andsize functionally desired, a dense isotropic pyrolytic carbon coatingcovering substantially all of the surface of said substrate which willcome in contact with blood, which pyrolytic carbon has a density of atleast about 1.5 grams per cc., a BAF between 1.0 and about 1.3 and anapparent crystallite size of less than about 200A. and means for director indirect connection of the device to the circulatory system of ahuman being.

2. A prosthetic device in accordance with Claim 1 wherein said pyrolyticcarbon has an apparent crystallite size of about 50A. or less.

3. A prosthetic device in accordance with Claim 2 wherein said pyrolyticcarbon has an apparent crystallite size of at least about 20A.

4. A prosthetic device in accordance with claim 1 wherein said pyrolyticcarbon coating is at least about 50 microns thick.

5. A prosthetic device in accordance with claim 4 wherein said substratehas no radius of curvature less than one-quarter inch and said pyrolyticcarbon has a BAF between 1.0 and about 2.0.

6. A prosthetic device in accordance with claim 1 wherein said pyrolyticcarbon has a thermal coefficient of expansion between about 3 and 6 X10'/ C. measured at about 20 C.

7. A prosthetic device in accordance with claim 6 wherein said pyrolyticcarbon has a thermal coefficient of expansion plus or minus about 50percent of the coefficient of thermal expansion of said substrate.

8. A prosthetic device m accordance with claim 1 wherein said pyrolyticcarbon contains a carbide additive dispersed therein.

9. A prosthetic device in accordance with claim 8 wherein said pyrolyticcarbon contains up to about 20 weight percent silicon in the form ofsilicon carbide.

10. A prosthetic device in accordance with claim 8 wherein saidpyrolytic carbon contains silicon in an amount between about 20 and 10weight percent in the form of silicon carbide.

11. A prosthetic device in accordance with claim 1 wherein saidsubstrate has the shape of an element of a blood circulatory assistdevice.

12. A prosthetic device in accordance with claim 1 wherein saidsubstrate has the shape of an element of an artificial heart valve.

13. A prosthetic device in accordance with claim 1 wherein saidsubstrate has the shape of a cannula for implantation into the body of ahuman being.

14. A prosthetic device in accordance with claim 1 wherein saidsubstrate is made of graphite.

v UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent 3,685,059Dated August 22, 1972 lnventofls) Jack C. Bokros and Willard H. Ellis Itis certified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 8, line 48, insert the word "cage" after "retaining" Column 8,line 52, change "in" to "is" Column 8, line 64, change "is to "are"Column 13, line 21, change "hat" to "that" On the cover page of thePatent Item [73] for "Gulf General Atomic Incorporated" read "GULF OILCORPORATION".

Signed and sealed this 15th day of March 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. 7 ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents.

JRM PO-105O (10-69)

2. A prosthetic device in accordance with Claim 1 wherein said pyrolyticcarbon has an apparent crystallite size of about 50A. or less.
 3. Aprosthetic device in accordance with Claim 2 wherein said pyrolyticcarbon has an apparent crystallite size of at least about 20A.
 4. Aprosthetic device in accordance with claim 1 wherein said pyrolyticcarbon coating is at least about 50 microns thick.
 5. A prostheticdevice in accordance with claim 4 wherein said substrate has no radiusof curvature less than one-quarter inch and said pyrolytic carbon has aBAF between 1.0 and about 2.0.
 6. A prosthetic device in accordance withclaim 1 wherein said pyrolytic carbon has a thermal coefficient ofexpansion between about 3 and 6 X 10 6/* C. measured at about 20* C. 7.A prosthetic device in accordance with claim 6 wherein said pyrolyticcarbon has a thermal coefficient of expansion plus or minus about 50percent of the coefficient of thermal expansion of said substrate.
 8. Aprosthetic device in accordance with claim 1 wherein said pyrolyticcarbon contains a carbide additive dispersed therein.
 9. A prostheticdevice in accordance with claim 8 wherein said pyrolytic carbon containsup to about 20 weight percent silicon in the form of silicon carbide.10. A prosthetic device in accordance with claim 8 wherein saidpyrolytic carbon contains silicon in an amount between about 20 and 10weight percent in the form of silicon carbide.
 11. A prosthetic devicein accordance with claim 1 wherein said substrate has the shape of anelement of a blood circulatory assist device.
 12. A prosthetic device inaccordance with claim 1 wherein said substrate has the shape of anelement of an artificial heart valve.
 13. A prosthetic device inaccordance with claim 1 wherein said substrate has the shape of acannula for implantation into the body of a human being.
 14. Aprosthetic device in accordance with claim 1 wherein said substRate ismade of graphite.